Liquid hydrogen

Liquid hydrogen is the liquid state of the element hydrogen. In nature, hydrogen is usually found in the molecular form of H2. It is a highly concentrated way to store hydrogen energy. Storing the element as a liquid takes up much less space than storing it as a gas. However, liquid hydrogen has a very low density. Its density is only 70.85 kg/m3 at 20 K. This makes its volumetric energy density much lower than other fuels.

To turn hydrogen into a liquid, it must be cooled significantly. It must reach a critical point of 33 K to exist as a liquid. To be fully liquid at atmospheric pressure, it must be cooled to -253 degrees Celsius. One common method for obtaining liquid hydrogen uses a compressor. This machine looks and works much like a jet engine. Once liquefied, the substance can be kept in thermally insulated containers. Even with this insulation, it can leak away at a rate of about 1% per day.

Hydrogen exists in two different spin isomers. These are called orthohydrogen and parahydrogen. In these molecules, the two nuclei can have different spin states. In orthohydrogen, the nuclear spins are parallel. In parahydrogen, the nuclear spins are antiparallel. At room temperature, 75% of hydrogen is orthohydrogen. However, parahydrogen is the more stable state. The lowest energy state for liquid hydrogen consists of 99.79% parahydrogen.

When hydrogen is cooled, orthohydrogen converts to parahydrogen. This is an exothermic reaction, which means it releases heat. This released heat can cause the liquid to boil off. To prevent this, workers use a catalyst during the liquefaction process. A catalyst is a substance that speeds up a chemical change. Common catalysts include iron(III) oxide, activated carbon, or nickel compounds. This converts the hydrogen to the parahydrogen form early. This prevents excessive boil-off during long-term storage.

Scientists have made many important discoveries regarding hydrogen. In 1885, Zygmunt Florenty Wróblewski published the critical temperature and pressure of hydrogen. In 1898, James Dewar successfully liquefied hydrogen. He used regenerative cooling and his invention, the vacuum flask. Later, in 1929, Paul Harteck and Karl Friedrich Bonhoeffer achieved the first synthesis of stable parahydrogen. These steps allowed us to understand and use liquid hydrogen in modern technology.

Liquid hydrogen is a vital fuel for rocketry. NASA and the U.S. Air Force use large liquid hydrogen tanks. Some of these individual tanks hold up to 3.8 million liters. In most rocket engines, the liquid hydrogen first cools the nozzle. Then, it is mixed with an oxidizer, which is usually liquid oxygen. They burn together to produce water, traces of ozone, and hydrogen peroxide. Many engines run fuel-rich to reduce nozzle erosion. This also increases the specific impulse of the engine.

Beyond rockets, liquid hydrogen has many other uses. It can power internal combustion engines or fuel cells. The MF Hydra ferry uses liquid hydrogen for power. Some submarines, like the Type 212, also use it. It is even being studied as a zero-carbon fuel for aircraft. Scientists also use it to cool neutrons for neutron scattering. This works because hydrogen nuclei and neutrons have similar masses. This allows for maximum kinetic energy exchange during collisions.

Safety is a major concern when handling liquid hydrogen. It is extremely cold, which can cause cold burns. It is also very flammable and can detonate when mixed with air. Recently vaporized hydrogen is very cold and heavier than air. This can create flammable mixtures near the ground. There is also a risk involving atmospheric oxygen. The liquid is so cold it can solidify oxygen on the outside of containers. This creates an oxygen-rich environment that can spontaneously ignite materials.